1. Technical Field
This invention relates to power conversion. More specifically, this invention relates to the cancellation of a common mode noise waveform.
2. Discussion of the Related Art
In a simple AC power distribution system, there are three wires in a cord connecting, for example, a computer to a wall socket, which distributes the AC power. There is an active wire, a neutral wire, and a ground wire. Common mode noise is present on both the active and neutral wires and may be measured with respect to ground, the term common refers to the fact that identical noise appears on both the active and neutral wires. In some situations, common mode noise may be created by lightning, circuit breakers switching, poor grounding, or use of surge protectors that divert noise from the neutral wires. In high-frequency switching mode power supplies, common mode noise is created by the high frequency switching device within the power supply.
Common mode noise presents a problem because the common mode noise attempts to dissipate its energy from neutral to ground or from active to ground. In switching-mode power supplies, common-mode noise may be coupled through a high-frequency transformer or along paths that have stray or parasitic capacitance. Under certain conditions, especially if the common-mode noise consists of high frequency impulses, there is a probability that the noise will see the high frequency transformer as a coupling capacitor and pass through the transformer unobstructed. The power supply may also act like a high-frequency radio antenna, which may result in the power supply not meeting electromagnetic interference (EMI) standards. In addition, in small form power supplies, more stray capacitance paths may exist simply because they are smaller in physical size and more densely packaged when compared to other power supplies.
If common mode noise is transferred through a switching-mode power supply, a noise voltage appears between the ground and the voltage-supply pins of the device being powered. If the noise exceeds the maximum voltage specification of the device being powered, the energy from the common mode noise may pass through the logic hardware to ground, dissipating energy along the way. Reduced reliability, interference with data processing, and permanent damage may result. The magnitude of the common-mode noise does not need to be high to cause damage because electronics components in the device being powered may be able to withstand only a few volts or a few tens of milliamperes of current.